Gwen would be a great character to pair with Alien X. Her magical abilities could complement Alien X's powers in interesting ways. For instance, when facing a threat that requires both brute omnipotent force and the finesse of magic. Another good pairing could be Kevin. His ability to absorb and use different forms of energy could interact with Alien X's power source in unexpected ways, like when they need to combine their powers to break through a powerful energy barrier.
Power and responsibility is also a big theme. Ben has to handle the powers of his alien forms responsibly, especially when dealing with aliens. In many fanfics, there's also the theme of discovery. Ben discovers new things about the aliens, their cultures, and their abilities. For example, he might learn a new power from an alien that he can use in a different way. There's also the theme of cooperation, where Ben and the aliens work together to achieve a common goal, like saving a planet or stopping a powerful enemy.
One idea could be exploring Alien X's internal conflict between Bellicus and Serena in a new situation. For example, when faced with a moral dilemma on a planet where two species are at war and both have valid reasons for their actions. Another plot could be Alien X having to deal with a powerful new enemy that can somehow disrupt his omnipotent powers. Maybe this enemy has the ability to split Bellicus and Serena's consciousness further, causing chaos within Alien X.
Heatblast is really cool. He can generate and control intense heat, which makes him great for combat and for melting things. He has a flaming body which gives him a very intimidating look.
Fourarms is really cool. He's big and strong with four arms, which gives him great power in combat. Heatblast is also amazing as he can generate and control fire.
Well, there's 'The Alien Bond'. In this fanfic, Ben Tennyson forms a very special bond with an alien that has the power to manipulate time. It's really interesting as it shows how their relationship develops and how they use their combined powers to face various challenges. There are also some fanfics that explore what would happen if Ben could communicate with his alien forms on a more personal level, like in 'Ben and the Sentient Aliens' where the aliens have their own personalities and thoughts that Ben has to deal with.
There are some really cool 'Ben 10 Alien Force Fanfiction' out there. 'The Mutant Invasion' is a story where mutants from another dimension invade Earth and Ben has to use his alien forms in unique combinations to fight them. 'Gwen's New Powers' is another interesting one. It shows Gwen developing some unexpected powers and how she and Ben deal with that. Also, 'Kevin's Redemption' is a story that focuses on Kevin trying to make up for his past mistakes.
I'm not entirely sure as it could be a very specific or fan - made story. But generally in the Ben 10 Ultimate Alien series, Ben Tennyson has a new Omnitrix which gives him access to ultimate forms of his aliens. The big story might involve him facing new villains, protecting the Earth and the galaxy, and dealing with the consequences of using his powers.
Some of the alien characters themselves, like Heatblast or Four Arms, are also popular. Their unique physical characteristics and powers make them interesting to write about. For instance, Heatblast's ability to control fire can lead to all kinds of scenarios, from battling fire - based villains to using his heat to solve environmental problems in a story.